1. Field of the Invention
This invention relates to a digital logic circuit, specifically to a digital logic circuit for use in an application-specific integrated circuit.
2. Description of the Prior Art
The progress in semiconductor technology allows computer systems to include more and more transistors without increase in size. In particular, computer systems originally disposed on a single circuit board referred as “System on Board” can now be accommodated on a single chip referred as “System on Chip” due to dimensional shrinkages of elements such as central processing units and memory components. Consequently, more and more System-on-Chip products are available on the market. Compared with systems disposed on circuit boards, systems accommodated in chips have the advantages of reduced costs, higher computing efficiency, and low power consumption and thus are becoming more and more popular. The smaller dimensions and lower costs allow System on Chips to be more versatile in potential uses, wherein System-on-Chips include application-specific integrated circuits (ASIC) that are tailored according to customer and electronic system requirements.
Application-specific integrated circuits (ASIC) have specific computing functions based on customer requirements. In the present semiconductor technology field, electronic design automation softwares are utilized to break up the application-specific integrated circuit (ASIC) into a plurality of logic elements with different logic functions such as AND, OR, XOR, or XNOR, wherein the electronic design automation software then arranges the logic elements in different places on the ASIC based on customer functionality requirements and size limitation of the application-specific integrated circuit.
FIG. 1 is a schematic view of an application-specific integrated circuit 10. The application-specific integrated circuit 10 includes a voltage rail 11, a ground rail 12, and a plurality of logic circuit rails 20, wherein the voltage rail 11 and the ground rail 12 are used to provide electrical signals. As FIG. 1 shows, the logic circuit rails 20 are electrically connected to the voltage rail 11 and the ground rail 12. The logic circuit rail 20 includes logic units 30 and auxiliary units 40 disposed next to each other. The logic units 30 are elements used to provide logic functions. Under the arrangement of electronic design automation software and under normal computing requirements, at least some logic circuit rails 20 consist of not only the logic units 30. Furthermore, the logic units 30 may be scattered over the logic circuit rail 20 due to design or circuitry requirements and therefore gaps may exist between the logic units 30.
The auxiliary units 40 illustrated in FIG. 1 are configured to fill the gaps between the logic units 30 and connect the logic units 30 to the voltage rail 11 and the ground rail 12. As FIG. 1 shows, the logic units 30 includes a logic voltage end 31 and a logic ground end 32 while the auxiliary unit 40 includes an auxiliary voltage end 41 and an auxiliary ground end 42. All the logic units 30 and the auxiliary units 40 are electrically connected to the voltage rail 11 via the logic voltage end 31 and the auxiliary voltage end 41, respectively. Similarly, the logic units 30 and the auxiliary units 40 are electrically connected respectively through the logic ground end 32 and the auxiliary ground end 42 to the ground rail 12 in order to establish an electrical loop with the voltage rail 11 and the ground rail 12 for electricity signal transmission.
However, the progress in semiconductor technology not only allows more and more transistors to be included in one logic unit 30, but also increase the number of logic units 30 in the application-specific integrated circuit 10. Consequently, connections between transistors are also becoming increasingly complex, which then cause the logic units 30 to increase in resistance, which in turn contributes to the increase in the overall resistance of the application-specific integrated circuit 10. The above-mentioned increase in resistance will influence the response time of the logic units 30 and thus the overall computing efficiency of the logic circuit rail 20. Furthermore, the width 50 of the logic voltage end 31 and the logic ground end 32 as well as their overall sizes are normally limited due to design choices. Therefore, how to reduce the overall resistance of the logic circuit rail 20 while maintaining the overall dimension of the logic unit 30 in a smaller scale is an important issue in the semiconductor technology.